在COUNT里面选择

Question

我想在COUNT里面嵌入一个SELECT，但是我找不到任何例子。

#pseudosql 
SELECT a AS current_a, COUNT(*) AS b, 
    COUNT(SELECT FROM t WHERE a = current_a AND c = 'const') as d, 
    from t group by a order by b desc 

Answer 1

你并不真的需要一个子选择：

SELECT a, COUNT(*) AS b, 
    SUM(CASE WHEN c = 'const' THEN 1 ELSE 0 END) as d, 
    from t group by a order by b desc 

Answer 2

您可以将内部的计数（）的子选择：

SELECT a AS current_a, COUNT(*) AS b, 
    (SELECT COUNT(*) FROM t WHERE a = current_a AND c = 'const') as d, 
    from t group by a order by b desc 

Answer 3

使用SELECT COUNT(*) FROM t WHERE a = current_a AND c = 'const') as d。

Answer 4

SELECT a AS current_a, COUNT(*) AS b, 
    (SELECT COUNT(*) FROM t WHERE a = current_a AND c = 'const') as d 
    from t group by a order by b desc